Is it possible to just block him/ghost out of nowhere? If they're toxic then they're not really worth giving a lot of energy to with an explanation for why you don't want to associate with them anymore. Getting enough space from someone usually helps tremendously in not caring about them anymore once you've cut them off. In the past I once sent someone a whole paragraph over why I was cutting them off purely to vent out my feelings but a different time I just randomly blocked the person on everything out of nowhere and it felt just as gratifying for me to do either method. Overall, I'm hoping the best for your situation!!

From my extremely limited experience and ignorance of the context to this, is it possible to just really whole heartedly embody petty avoidance? Like, getting really into ensuring you're at least 10ft away at all times and not speaking to them or making eye contact? It's very much toddler behaviour, but focusing on every situation like a game, in a sense, might make it easier for you to not think about how shitty the person makes you feel, saying something unfortunate or something they can use against you, or prevent them from "convincing" you to stick around? (This only works with moderately-mostly sane people. If you think they'll flip out and start following you around or something, maybe don't do this)